Intercalation is one of several modes by which drugs interact with DNA wherein a planar portion of the drug is inserted in between adjacent stacked base pairs of a double stranded DNA. The intercalation process results in helix extension and unwinding of the DNA. Included within these drugs are antitumor agents, actinomycin D, adriamycin and daunomycin, as well as several drugs for treatment of parasitic disease including ethidium bromide, quinacrine, chloroquine and miracil D. U.S. Pat. No. 2,441,665 discloses a class of alkylene diamine derivatives which are valuable as antimalarial agents. U.S. Pat. No. 2,113,357 discloses basically substituted amino-acridine derivatives useful in treating blood parasites.
More recently, DNA intercalating ligands have been proposed for use in targeting alkylating agents to DNA by attachment of the intercalating ligand to the alkylating agent. Gourdie et al. J. Med. Chem. 1991 34:240-248.
Since the biological properties of these DNA intercalating drugs are believed to result from their binding, researchers have focused efforts on designing molecules that have a high affinity for DNA. Planar polycyclic aromatic molecules show a strong propensity to bind to DNA by intercalation. Jaycox et al. J. Heterocyclic Chem. 1987 24:1405-1408. In recent years, efforts to identify molecules with a greater affinity and selectivity for DNA have resulted in the development of bifunctional intercalating agents in which two intercalating ligands are bridged by a central linking chain. In general, enhanced binding is observed with molecules of this type. Canellakis et al. Biochim. Biophys. Acta, 1976 418:277; Becker and Dervan, J. Am. Chem. Soc. 1979 101:3664; and Wakelin et al. Med. Res. Rev. 1986 6:275. However, the chemical and physical nature of the linking chain has been found to play a major role in the binding process.
A homologous series of diacridines containing two 9-aminoacridine chromophores linked via a simple methylene chain were studied in order to investigate the minimum interchromophore separation required to permit bifunctional intercalation. Wakelin et al. Biochemistry 1978 17:5057. Viscometric, sedimentation and electric dichroism experiments showed that compounds having one to four methylene groups in the linker are restricted to monofunctional intercalation whereas bifunctional interaction was observed when the chain length was increased to six methylene groups or more. Accordingly, the transition in functionality was concluded to lie between interchromophore distances of 6.3 and 8.8 angstroms.
Additional studies however, indicate that the nature of the bridging chain and/or substituents on the acridine ring has a profound effect on the ability of diacridine compounds to act as bifunctional intercalating agents. DNA binding characteristics of a number of acridine dimers of which two aromatic rings, each ring being the monomeric 2-methoxy-6-chloro-9-(3-dimethyl amino propylamino acridine), were linked by a chain of varying length and structure were also determined . Le Pecq et al. Proc. Nat'l Acad. Sci. USA 1975 72(8):2915-2919. The linking chains of these acridine dimers include: --(CH.sub.2).sub.3 --NH--(CH.sub.2).sub.4 --NH--(CH.sub.2).sub.3 --(IIA); --(CH.sub.2).sub.3 --NH--(CH.sub.2).sub.4 (IIB); and (CH.sub.2).sub.3 --NH--(CH.sub.2).sub.3 (III). It was found that the 2 dimers with the longest chain length, (IIA) and (IIB), bisintercalated while only one of the two rings of the dimer with the shortest chain (III) of 9 angstroms was intercalated. Bisintercalative binding of a bis(acridine) derivative specifically designed as a relatively rigid molecule in which the two chromophores are essentially coplanar and 7 angstroms apart in the only reasonably strain-free conformation has also been reported. Atwell et al. J. Am. Chem. Soc. 1985 107:4335-4337.
In contrast, bis-intercalators bridged by flexible chains have generally exhibited reduced affinities for DNA, in part because of self stacking interactions which compete with the binding process. Barbet et al. Biochemistry 1976 15:2642; and Capelle et al. Biochemistry 1979 18:3354. Further, bis-intercalation can introduce undesirable entropic effects when a flexible linker is forced into an extended chain conformation. Jaycox et al. J. Heterocyclic Chem. 1987 24:1405-1408. In addition, it is a concern that flexible bis-intercalators can creep in a stepwise fashion along the DNA macromolecule, thereby lowering ligand residence lifetimes at any one site. Denny et al. J. Med. Chem. 1985 28:1568. Such a process could have significant effects on efficacy of these intercalators as anticancer agents as residence lifetimes have been correlated with in vivo antitumor activity for a large number of DNA intercalators. Feigon et al. J. Med. Chem. 1984 27:450. Studies by Jaycox et al. indicate that it is the rigid tether framework and not the aromatic character of the linking chain that is responsible for the cytotoxic action of DNA intercalating agents. J. Heterocyclic Chem. 1987 24:1405-1408.
In the present invention a series of novel bis-acridinyl DNA intercalating agents with semi-flexible linking chains have been prepared and demonstrated to be tight binders of DNA and to have antitumor activity.